Cap for a writing instrument



FIG. 1 is a front elevational view of a cap for a writing instrument showing my new design;

FIG. 2 is a left side elevational view thereof, the right side being the same in appearance as the left side;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a cap for a writing instrument, as shown and described. 